1. Field of the Invention
The present invention relates to a conductive paste and a conductive paste curing method, to a method of forming an antenna for a radio frequency identification medium employing the conductive paste, and to a radio frequency identification medium.
2. Background Art
Recent years have seen the development of radio frequency identification media such as so-called non-contact IC cards for use as ID cards, tags, name tags and the like, that allow reading out and writing in of data using read/write devices even at distances of 4-5 meters. Such non-contact IC cards are disclosed in Japanese Patent Application Laid-open No. 5-166018, Japanese Patent Application Laid-open No. 8-216570 and elsewhere. This type of radio frequency identification media is formed comprising a substrate and a coating member, and sandwiched therebetween an antenna for transmitting and receiving data utilizing electromagnetic waves as the transmission medium, with an IC chip having built-in functions allowing writing and storage of the data, as well as write-over and erasing.
A radio frequency identification medium, being typically just such a non-contact IC card, requires the antenna connected to the IC chip to have a thin shape. Methods employed to form such antennas include vapor deposition methods whereby a metal is directly vapor deposited onto a substrate and etching methods whereby a metal thin-layer is preformed on a substrate and then etched, as well as coil bonding methods whereby a separately formed coil is bonded to a substrate, and printing methods whereby a conductive paste is printed onto a substrate in an antenna shape to form the antenna.
Of these antenna-forming methods, the highest sensitivity thereof is exhibited by antennas formed by metal vapor deposition methods. However, when irregularities are present on the surface of the substrate during vapor deposition of the metal on the surface of the substrate, the surface condition has a direct influence to the deposition surface, resulting in irregularities on the metal vapor deposition surface. This has led to inconveniences including greater turbulence of the electromagnetic field distribution and lower sensitivity of the antenna.
Etching methods are antenna-forming methods that allow easy formation of any desired antenna shape. However, they have required many production steps including coating of the resists, patterning for the resists, and etching with etching solutions. In addition, the expensive equipment and working environment inevitably raises manufacturing costs. Another requirement is safe disposal of the large amounts of waste liquids generated during the etching step.
Antennas formed by coil bonding methods are superior in terms of sensitivity. However, workability therefor has been poor because the metal coil is bonded directly to the surface of the substrate. There are also many difficulties involved in reliably bonding metal coils to substrates, and therefore productivity has been inferior.
In this regard, the printing methods mentioned as above can easily form any desired shape of antenna by printing techniques, and they also allow satisfactory workability. Specifically, the productivity of the various manufacturing steps such as screen printing of conductive pastes, bonding of IC chips and antennas and attachment of coating sheets (or film lamination) can be improved by the performance of the printing machines themselves.
Currently, conductive layer formation for these antennas and the like is mostly carried out using thermosetting conductive pastes that are cured by heat. Such thermosetting conductive pastes usually contain solvents, and the performance is exhibited by evaporation of the solvent or by evaporation of the solvent coupled with curing of a binder resin. A phenol resin, polyester resin, or the like is used as the binder resin.
However, conductive layer formation with thermosetting conductive pastes requires heating at 150.degree. C. or above and treatment times on the order of half an hour or longer. The processing time for formation of conductive layer formation for antennas, etc. is therefore longer. Furthermore, deterioration of the substrates on which the conductive layers are formed has also been unavoidable.
On the other hand, photosetting conductive pastes are also used for conductive layer formation. When photosetting conductive pastes are used, it is necessary to remove organic matters by sintering as post-treatment after the photosetting in order to improve the conductivity of the antenna. Thus, once the photosetting conductive paste has been fixed onto the substrate by light irradiation, the organic matters are decomposed and removed by post-heating at 500.degree. C. or above to form the conductive layer. This post-heating is usually carried with an electric furnace. However, as mentioned above, since heating is carried out at 500.degree. C. or above for the post-heating, only heat resistant materials such as ceramics can be used for the substrate on which the conductive layer is formed of the photosetting conductive paste. Consequently, materials with poor heat resistance, such as paper and plastics, are not suitable as the substrates.
Further, when a photosetting conductive paste is used, formation of hard cured products results in brittleness and poor bending resistance. Also, although the monomers have high reactivity, there is notable bleed-out after application, and while their oligomers can soften the cured products, their high viscosity during application makes it difficult to part them from plates, and thus it has been difficult to set the balance for application.